1. In a small bowl, cover porcini with 1 cup warm water. Let soak, stirring occasionally, 20 minutes. Using a slotted spoon, remove porcini; gently squeeze dry with hands. Reserve soaking liquid. Coarsely chop porcini, and set aside.

2. Preheat oven to 375 degrees. In a stockpot over medium heat, melt 2 tablespoons butter. Add leeks; cook, stirring occasionally, until very soft and translucent, about 7 minutes. Add wine; cook until evaporated. Add chard; cook, stirring, until chard is wilted and bright green, about 2 minutes. Add stock and 1/2 cup reserved soaking liquid, leaving behind any sediment. Bring to a simmer.

3. Meanwhile, in a large saute pan, melt 2 tablespoons butter over medium-high heat. Add half the mushrooms (including reserved porcini); season with salt and pepper. Cook, stirring occasionally, until the mushrooms are golden brown. Transfer to the pot with the stock and vegetables. Repeat with the remaining butter and mushrooms. Stir in thyme.

4. To assemble, cover the bottom of a 5-quart Dutch oven with toasted bread slices. Pour half of the stock and vegetables over the bread. Top with a single layer of squash slices, followed by more of the stock and vegetables, and then another layer of squash slices. Pour any remaining stock over the entire dish.

5. Cover pot with lid, and bake until the squash is tender when pierced with a paring knife, about 1 hour. Remove from oven, and let cool slightly before serving.